Ancient Loch

The rocks to the east of the river represent as significant contrast to those in the west. East of the river the rocks form part of the same sequences that has resulted in the flat lying nature of Caithness. These rocks are termed Caithness Flagstones and were formed in a series of fresh-water lochs during the Devonian geological period, often terms Lake Orcadie. The series of lochs extended over Caithness, Orkney, Shetland, western Norway and the Moray coast.
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In northern Scotland the Devonian succession is divided into the Lower, Middle and Upper Old RedSandstone. These divisions roughly translate into the Lower, Middle and Upper Devonian. Often these units are separated by breaks in sedimentation with associated erosion (called unconformities) and represent periods of uplift in the underlying ground level.
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In Strathy and Baligill the rocks belong to the Lower and Middle Devonian subdivision and have been termed the Baligill Outlier.
The Lower Devonian was deposited on the irregular basement of post-Caledonian land surface. Before these rocks were deposited there was a long period of erosion and weathering. Mountains and hills to the west were cut by rivers and alluvial fans in a fairly hot and arid climate. Flash floods resulted in dumps of sandy material on the (sometimes dried out) lake floor. The area of the Baligill Outlier represents the westernmost margin of the Orcadian lake system.

The Middle Devonian lithologies were deposited in the main period of lake development. The rocks are predominantly grey or green in colour and form a series of cyclical deposits recording deposition in a lake of fluctuating depth and extent.
During deep lake periods dark organic-rich laminated beds were deposited. These contain the fossil remains of boney-headed fish that swam in the lake. The fish beds, as they are termed, are correlated to the fish-bearing layer at Achanarras near Spittal in Caithness.

From the slipway at Port Skerra one can see in the cliff face opposite the relationship between the Devonian rock are the underlying basement. The irregular surface of gneiss is overlain unconformably by conglomerates and sandstones probably representing the deposits of flash floods.
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In Baligill, just north of the bridge crossing the stream, a grey carbonate laminite is exposed in the stream bank and is overlain by a asandstone. This is a typical basin margin feature and represents the transition from deep lake to shallow conditions without any transitional lithologies in between. The sandstone fines up into thin bedded flags and a second carbonate laminate sits on top. Both the laminites contain the fossil fragments of the fish that once swam these waters.
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Further down stream near the old lime kilns a laminated limestone (thus the lime kilns!) is overlain by a green shales and sandstones. Part of this sequence has been interpreted as having been deposited in moderately deep water by deep currents. The inclusion of granite pebbles indicates that the basement was located nearby. The uppermost sandstone is a shallow water deposit from a river or river-delta origin deposited during a period of low lake level.
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At Baligill Quarry two fish-bearing lacustrine laminites are exposed. The organic-rich beds also contain lenses of sand. These were probably rafted into the deep lake environment by being attached to patches of floating algal or bacterial mats. The fauna of the fish–beds contains Gyroptychius, Osteolepis and Palaeospondylus.